The ultrasonic echolocation calls produced by bats contain changing frequency and amplitude information through time. The accurate measurement of this information is desirable for automated species identification and other applications. Conventionally, two well known techniques for analyzing echolocation calls, each with their own limitations, are used.
One technique for analyzing echolocation calls includes digitally sampling the ultrasonic signals using an analog-to-digital converter, and then using a series of Fourier transforms to measure the power distribution through discrete frequency bins changing through time. This technique is known as full spectrum analysis. The Fourier transformation takes as input a sequence of N input samples at a sample rate R to produce N/2 evenly spaced frequency bins between 0 and R/2. For a larger value of N, the frequency resolution can be improved by dividing the power spectrum into a larger number of frequency bins, but at the cost of temporal resolution because more samples are required. Conversely, the temporal resolution can be improved by using a smaller value of N, but at the cost of frequency resolution because fewer frequency bins are used.
The second technique for analyzing echolocation calls is to use a comparator to convert the ultrasonic signal into a square wave with each transition representing a zero crossing from either negative to positive or positive to negative. A high frequency digital counter can be used to measure the zero crossing periods with high precision. Because most echolocation calls are narrowband frequency modulated signals, the zero crossing technique can accurately measure the change of frequency through time. However, all amplitude information is lost and thus the power spectrum cannot be determined by the zero crossing technique.